Optical fiber and method of making the same

ABSTRACT

An optical fiber is composed of silica glass and comprises a center core region doped with F element, a ring core region doped with GeO 2 , and an inner cladding region doped with F element; wherein a buffer layer composed of undoped SiO 2  or SiO 2  doped with one or both of P and Cl or a concentration gradient region in which GeO 2  concentration radially decreases toward the boundary is provided between the center core region and the ring core region.

This application is a divisional of Application Serial No. 09/466,798,filed Dec. 20, 1999 now U.S. Pat. No. 6,519,403 issued Feb. 11, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber having a ring coreportion, and a favorable method of making the same.

2. Related Background Art

Dispersion-shifted optical fibers have a zero-dispersion wavelength,where their chromatic dispersion value becomes zero, in the wavelengthrange of 1.55 μm. Known as one kind thereof are those of a ring-shapedstructure in which a ring core region having a higher refractive indexand a cladding region having a lower refractive index are disposedconcentrically around a center core region. A dispersion-shifted opticalfiber having such a ring-shaped structure of refractive index profile ismanufactured by drawing an optical fiber preform having a similarrefractive index profile.

The refractive index profile of this optical fiber preform can berealized by employing silica glass as a main ingredient, adding F(fluorine) element to a center core portion which is to become thecenter core region of the optical fiber, and adding GeO₂ (germaniumdioxide) to a ring core portion which is to become the ring core region.This optical fiber preform is subjected to melt spinning, i.e.,so-called drawing, an optical fiber having a desirable refractive indexprofile may be obtained.

SUMMARY OF THE INVENTION

However, upon drawing, the optical fiber preform is heated and, at thistime, the F element added to the center core portion diffuses into itssurrounding regions, whereas Ge added to the ring core portion diffusesinto the center core portion and the outer cladding region. Due to thismutual diffusion, transmission loss would increase. Also, when Ge and Fmingle with each other, then GeF₄ or GeO may be produced in a heatingand unifying process, so as to generate bubbles, which deteriorate thequality of the optical fiber to be made. As a result, there has been aproblem that desirable fiber characteristics may not be obtained.

In order to overcome the above-mentioned problem, it is an object of thepresent invention to provide an optical fiber of a ring-shaped structurehaving a region doped with F element and a region doped with Ge, whichcan be manufactured stably and has a desirable refractive index profile;and a method of making the same.

For overcoming the above-mentioned problem, the optical fiber inaccordance with the present invention is composed of silica glass,having a center core region doped with F element and a ring core regiondoped with GeO₂, wherein a buffer layer made of undoped SiO₂ (silicondioxide) or SiO₂ doped with both or one of P (phosphorus) and Cl(chlorine) is disposed between the center core region and the ring coreregion.

The optical fiber in accordance with the present invention can be madeby drawing an optical fiber preform having a cross-sectional profilesimilar to that of this optical fiber. Namely, the optical fiber preformalso has a buffer layer made of undoped SiO₂ or SiO₂ doped with one orboth of P and Cl. When this optical fiber is drawn, the mutual diffusionof F and Ge between the ring core region and the center core region issuppressed due to the existence of buffer layer. The fact that thisbuffer layer exists in the resulting optical fiber indicates that themutual dispersion suppressing effect sufficiently functions, so as tomaintain fiber characteristics.

Preferably, the thickness of the buffer layer is at least 0.01 μm butnot greater than 5 μm. If the buffer layer is thinner than the lowerlimit of this range, there is a possibility of the mutual diffusion ofGe and F occurring beyond the buffer layer. On the other hand, if thebuffer layer is thicker than the upper limit of the range, the bendingloss occurring when the optical fiber is bent may become unfavorablylarge.

Alternatively, the optical fiber in accordance with the presentinvention is composed of silica glass, having a center core region, aring core region doped with GeO₂ (germanium dioxide), and an innercladding region doped with F (fluorine) element which are arrangedconcentrically, wherein a buffer layer made of undoped SiO₂ or SiO₂doped with both or one of P and Cl is disposed between the ring coreregion and the inner cladding region.

The mutual diffusion of F and Ge between the ring core region and theinner cladding region when drawing an optical fiber preform having asimilar structure is suppressed due to the existence of buffer layer inthis case as well. Similarly, the fact that this buffer layer exists inthe resulting optical fiber indicates that the mutual dispersionsuppressing effect sufficiently functions, so as to maintain fibercharacteristics.

In this case, the thickness of the buffer layer is preferably 0.01 μm orgreater. It is because of the fact that there is a possibility of themutual diffusion of Ge and F occurring beyond the buffer layer if thebuffer layer is thinner than this lower limit. On the other hand, thechange in bending loss occurring upon bending the optical fiberdepending on the thickness of the buffer layer in this case is smallerthan that in the case where the buffer layer is formed between the ringcore region and the center core region, whereby the buffer layer can bemade thicker.

Alternatively, the optical fiber in accordance with the presentinvention is comprised of silica glass, having a center core regiondoped with F element and a ring core region doped with GeO₂; wherein,letting a [μm] be the radius of the center core region, and C_(G)(r) [wt%] be the concentration of GeO₂ in the ring core region at a positionseparated from the center by a radius r [μm], the concentration gradientY_(G1) [wt %·μm²] of GeO₂ in a boundary portion of the ring core regionwith respect to the center core region defined by: $\begin{matrix}{y_{G1} = {\int_{a}^{a + 1}{\left( {{{rC}_{G}(r)}{\exp \left( {a - r} \right)}} \right){r}}}} & (1)\end{matrix}$

is set to 100 wt %·μm² or less; or, letting C_(F)(r) [wt %] be theconcentration of F element in the center core region at a positionseparated from the center by a radius r [μm], the concentration gradienty_(F1) [wt %·μm²] of F element in a boundary portion of the center coreregion with respect to the ring core region defined by: $\begin{matrix}{y_{F1} = {\int_{a - 1}^{a}{\left( {{{rC}_{F}(r)}{\exp \left( {r - a} \right)}} \right){r}}}} & (2)\end{matrix}$

is set to 18 wt %·μm² or less.

Alternatively, the optical fiber in accordance with the presentinvention is composed of silica glass, having a center core region, aring core region doped with GeO₂, and an inner cladding region dopedwith F element which are arranged concentrically; wherein, letting b[μm] be the radius of the ring core region, and C_(G)(r) [wt %] be theconcentration of GeO₂ in the ring core region at a position separatedfrom the center by a radius r [μm], the concentration gradient Y_(G2)[wt %·μm²] of GeO₂ in a boundary portion of the ring core region withrespect to the inner cladding region defined by: $\begin{matrix}{y_{G2} = {\int_{b - 1}^{b}{\left( {{{rC}_{G}(r)}{\exp \left( {r - b} \right)}} \right){r}}}} & (3)\end{matrix}$

is set to 180 wt %·μm² or less; or, letting C_(F)(r) [wt %] be theconcentration of F in the inner cladding region at a position separatedfrom the center by a radius r [μm], the concentration gradient Y_(F2)[wt %·μm²] of F element in a boundary portion of the inner claddingregion with respect to the ring core region defined by: $\begin{matrix}{y_{F2} = {\int_{b}^{b + 1}{\left( {{{rC}_{F}(r)}{\exp \left( {b - r} \right)}} \right){r}}}} & (4)\end{matrix}$

is set to 30 wt %·μm² or less.

The diffusion velocity of the above-mentioned mutual diffusion of Fe andGe generated upon drawing results from the concentration gradient in theboundary portion between the region doped with F element and the regiondoped with GeO₂. The inventors have found that, when the radialdistribution of doping amount of GeO₂ or F element in each region is setsuch that the weighted concentration gradients y_(G1), y_(F1), y_(G2),y_(F2) in boundary portions defined by equations (1) to (4) becomepredetermined values or less, the diffusion velocity of F and Ge lowers,thereby suppressing the mutual diffusion. As a consequence, desirablefiber characteristics are reliably obtained.

On the other hand, the method of making an optical fiber in accordancewith the present invention comprises steps of: (1) preparing a silicaglass pipe having a layer doped with GeO₂ at least on an inner peripheryside thereof; (2) depositing undoped SiO₂ or SiO₂ doped with one or bothof P and Cl onto the inside of the silica glass pipe, so as to produce abuffer layer; (3) inserting a silica glass rod doped with F element intothe inside of the buffer layer, and then heating and unifying the silicaglass rod and the buffer layer, so as to prepare an intermediatepreform; and (4) melt-spinning an optical fiber preform including theintermediate preform. According to this method, an optical fiber havingthe above-mentioned buffer layer can be made favorably.

Alternatively, the method of making an optical fiber in accordance withthe present invention comprises steps of: (1) preparing a silica glasspipe having a layer doped with GeO₂ at least on an inner periphery sidethereof; (2) heating the silica glass pipe, so as to evaporate GeO₂ inits inner surface and eliminate at least a part thereof; (3) inserting asilica glass rod doped with F element into the inside of the silicaglass pipe, and then heating and unifying the silica glass rod and thesilica glass pipe, so as to prepare an intermediate preform; and (4)melt-spinning an optical fiber preform including the intermediatepreform. According to this method, the above-mentioned optical fiber inwhich the germanium dioxide concentration in the vicinity of theboundary portion of the ring core region is lowered can be madefavorably.

The method of making an optical fiber having an inner cladding region inaccordance with the present invention includes the following threemethods. The first method comprises steps of: (1) preparing a silicaglass pipe having a layer doped with F at least on an inner peripheryside thereof; (2) depositing undoped SiO₂ or SiO₂ doped with one or bothof P and Cl onto the inside of the silica glass pipe, so as to form abuffer layer; (3) further forming at an inner peripheral portion thereofa glass layer doped with GeO₂, so as to produce an intermediate pipe;(4) inserting a silica glass rod into the inside of the intermediatepipe, and then heating and unifying the silica glass rod and theintermediate pipe, so as to prepare an intermediate preform; and (5)melt-spinning an optical fiber preform including the intermediatepreform.

The second method comprises steps of: (1) concentrically forming,successively from the axial center side, a silica layer, a layer dopedwith GeO₂, and an undoped layer or a layer doped with one or both of Pand Cl, so as to prepare a silica glass rod; (2) preparing a silicaglass pipe having a layer doped with F element at least on an innerperiphery side thereof; (3) inserting the silica glass rod into thesilica glass pipe, and then heating and unifying the silica glass rodand the silica glass pipe, so as to prepare an intermediate preform; and(4) melt-spinning an optical fiber preform including the intermediatepreform.

The third method comprises the steps of (1) and (2) steps of the firstmethod; (3) forming a silica glass rod having a silica layer at theaxial center and a layer doped with germanium dioxide therearound; (4)inserting the silica glass rod into the inside of the buffer layer, andthen heating and unifying the silica glass rod and the buffer layer, soas to prepare an intermediate preform; and (5) melt-spinning an opticalfiber preform including the intermediate preform. According to any ofthese methods, the optical fiber in accordance with the presentinvention having a buffer layer between the inner cladding region andthe ring core region can be made favorably.

In the step of preparing the silica glass pipe in these methods, asilica glass rod having a layer doped with fluorine element or germaniumdioxide at least on the inner periphery side thereof may be plasticallydeformed at a high temperature, so as to form an opening penetratingthrough the axial center, thereby yielding a glass pipe. Hence, arequired silica glass pipe can be formed favorably.

Another method of making an optical fiber in accordance with the presentinvention comprises steps of: (1) preparing a silica glass rod having asilica layer on the axial center side and a ring-shaped layer doped withgermanium dioxide disposed on the outside thereof; (2) opening thesilica glass rod along the axis, so as to prepare a silica glass pipe;(3) inserting a silica glass rod doped with fluorine element into theinside of the silica glass pipe, and then heating and unifying thesilica glass rod and the silica glass pipe, so as to form anintermediate preform; and (4) melt-spinning an optical fiber preformincluding the intermediate preform. The above-mentioned optical fiberhaving a buffer layer can favorably be made by this method as well.

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lateral sectional view showing a structure of a firstembodiment of the optical fiber in accordance with the presentinvention, whereas

FIG. 2 is a chart showing the refractive index profile thereof;

FIG. 3 is a flowchart of a first method of making the optical fiber ofFIG. 1, whereas

FIGS. 4A to 4H are lateral sectional views showing the respectiveintermediate products at individual steps thereof;

FIG. 5 is a flowchart of a second method of making the optical fiber ofFIG. 1, whereas

FIGS. 6A to 6F are lateral sectional views showing the respectiveintermediate products at individual steps thereof;

FIG. 7 is a flowchart of a third method of making the optical fiber ofFIG. 1, whereas

FIGS. 8A to 8F are lateral sectional views showing the respectiveintermediate products at individual steps thereof;

FIGS. 9A to 9C are longitudinal sectional views showing respective stepsof a method of making a silica glass tube;

FIG. 10 is a graph showing relationships between the thickness of aninner buffer layer and the transmission loss in optical fibers, whereas

FIG. 11 is a graph showing relationships between the thickness of theinner buffer layer and the bending loss in optical fibers;

FIG. 12 is a graph showing relationships between the thickness of anouter buffer layer and the transmission loss in optical fibers, whereas

FIG. 13 is a graph showing relationships between the thickness of theouter buffer layer and the bending loss in optical fibers;

FIG. 14 is a lateral sectional view showing a structure of a secondembodiment of the optical fiber in accordance with the presentinvention, whereas

FIG. 15 is a chart showing the refractive index profile thereof;

FIG. 16 is a flowchart of a method of making the optical fiber of FIG.14, whereas

FIGS. 17A to 17F are lateral sectional views showing the respectiveintermediate products at individual steps thereof;

FIG. 18 is a graph showing relationships between the GeO₂ concentrationgradient y_(G1) in the inner boundary of a ring core region and thetransmission loss in optical fibers;

FIG. 19 is a graph showing relationships between the GeO₂ concentrationgradient y_(G2) in the outer boundary of the ring core region and thetransmission loss in optical fibers;

FIG. 20 is a chart showing the refractive index profile of a thirdembodiment of the optical fiber in accordance with the presentinvention;

FIG. 21 is a graph showing relationships between the F elementconcentration gradient y_(F1) in the outer boundary of a center coreregion and the transmission loss in optical fibers;

FIG. 22 is a chart showing the refractive index profile of a fourthembodiment of the optical fiber in accordance with the presentinvention;

FIG. 23 is a graph showing relationships between the F elementconcentration gradient y_(F2) in the inner boundary of an inner claddingregion and the transmission loss in optical fibers; and

FIGS. 24A and 24B are charts showing respective refractive indexprofiles of other embodiments of the optical fiber in accordance withthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, preferred embodiments of the present invention will beexplained with reference to the accompanying drawings. To facilitate thecomprehension of the explanation, the same reference numerals denote thesame parts, where possible, throughout the drawings, and a repeatedexplanation will be omitted.

FIG. 1 is a view showing a cross-sectional structure of a firstembodiment of the optical fiber in accordance with the presentinvention, whereas FIG. 2 shows the refractive index profile thereof.

The optical fiber in accordance with the first embodiment is made ofsilica glass, and is constituted, as shown in FIGS. 1 and 2, by a centercore region 11 (having an outside diameter 2 a), an inner buffer layer12 (having a thickness t₁), a ring core region 13 (having an outsidediameter 2 b and a thickness t_(b)), an outer buffer layer 14 (having athickness t₂), an inner cladding region 15 (having an outside diameter 2c and a thickness t_(c)), and an outer cladding region 16, all of whichare concentrically arranged successively from the center. Among them,both of the inner and outer buffer layers 12, 14 and the outer claddingregion 16 are made of pure silica including substantially no additivesexcept for P or Cl and have a refractive index n₀, the center coreregion 11 is doped with an adjusted amount of fluorine element so as toyield a refractive index n₁ which is lower than n₀, the ring core region13 is doped with an adjusted amount of germanium dioxide so as to yielda refractive index n₂ which is higher than n₀, and the inner claddingregion 15 is doped with fluorine element so as to yield a refractiveindex n₃ which is lower than n₀.

The outside diameter 2 a of the center core region 11 is a fewmicrometers, the outside diameter 2 b of the ring core region 13 is onthe order of a few micrometers to 10 μm (the thickness t_(b) being abouta few micrometers), the outside diameter 2 c of the inner claddingregion 15 is a few micrometers to a few tens of micrometers (thethickness tc being on the order of a few micrometers to 10 μm, and theoutside diameter of the outer cladding region 16 is normally 125 μm.Also, each of the respective thicknesses t₁, t₂ of the buffer layers 12,14 is on the order of 0.01 to 5 μm. The relative refractive indexdifference Δn⁻=(n₁−n₀)/n₀ of the center core region 11 with reference tothe refractive index n₀ of the outer cladding region 16 is −0.2% to−0.7%, and the relative refractive index difference Δn⁺=(n₂−n₀)/n₀ ofthe ring core region 13 with reference to the refractive index n₀ of theouter cladding region 16 is on the order of 0.5% to 1.5%.

Two kinds of different methods among methods of making this opticalfiber will now be explained by way of example. FIG. 3 is a flowchart ofa first method of making the optical fiber of FIG. 1, whereas FIGS. 4Ato 4H are lateral sectional views showing the respective intermediateproducts at individual steps thereof.

First, at step S11, a starting glass tube 10 made of silica glass dopedwith F element is prepared (FIG. 4A). This starting glass tube 10 is tobecome an inner cladding region 15 having a low refractive index in theoptical fiber, and has a tubular form with an inside diameter of 14 mm,an outside diameter of 25 mm, and a length of 300 mm, for example. Here,the concentration of doping F element is preferably on the order of 0.4wt % to 1.5 wt %.

At step S12, undoped SiO₂ or SiO₂ doped with one or both of P and Cl isdeposited with a thickness of about 0.06 mm on the inner peripheral faceof the starting glass tube 10 by a CVD technique, so as to form theouter buffer layer 14 (FIG. 4B). Adding P or Cl to the outer bufferlayer 14 adjusts the viscosity differences between this layer and thelayers formed on both sides thereof, so that the drawing can stably becarried out when making the optical fiber. Subsequently, at step S13,SiO₂ doped with GeO₂ is deposited with a thickness of about 1.7 mm onthe inner peripheral face of the outer buffer layer 14 by the CVDtechnique, so as to form the ring core region 13 (FIG. 4C). Theconcentration of GeO₂ added here is preferably on the order of 10 wt %to 30 wt %. As a consequence, the ring core region 13 attains a highrefractive index. Further, at step S14, undoped SiO₂ or SiO₂ doped withone or both of P and Cl is deposited with a thickness of about 0.04 mmon the inner peripheral face of the ring core region 13 by the CVDtechnique, so as to form the inner buffer layer 12 (FIG. 4D). As aresult, a multilayer glass tube 10 having an inside diameter of about12.2 mm is obtained.

Adding P or Cl to silica glass forming the buffer layers 12, 14 canadjust the viscosity of each layer at the time of heating and melting,whereby the heating and drawing explained later can be carried outeasily and reliably.

At step S15, a silica glass rod 11 a doped with F element, which is tobecome the center core region 11, is inserted into the inside of thusformed multilayer glass tube 10 (FIG. 4E). Here, the F elementconcentration in the center core region 11 is on the order of 0.5 wt %to 2.5 wt %. At this time, a gap may be formed between the multilayerglass tube 10 and the silica glass rod 11 a. However, for yielding anoptical fiber whose core region has a low ellipticity, the gap ispreferably as small as possible. Also, before the insertion, both or oneof the multilayer glass tube 10 and the silica glass rod 11 a may besurface-treated with an HF solution or heated and extended until anappropriate diameter is attained. In the case where an oxygen/hydrogenflame is utilized for heating and extending the silica glass rod 11 a,the surface treatment with the HF solution is essential for eliminatingthe moisture attached to the surface of the silica glass rod 11 a.

At step S16, the multilayer glass tube 10 in the state where the silicaglass rod 11 a is inserted therein is heated under a reduced pressure soas to unify them together, whereby a multilayer silica glass rod 20 isobtained (FIG. 4F). This heating and unifying step is carried out in anatmosphere of Cl₂ gas or a mixed gas made of Cl₂ gas and O₂ gas.

At step S17, this multilayer silica glass rod 20 is inserted into theinside of a pure silica glass tube 16 a (FIG. 4G). This pure silicaglass tube 16 a is to become the outer cladding region 16 having a lowrefractive index in the optical fiber. Here, before the insertion, themultilayer silica glass rod 20 may be surface-treated with an HFsolution or heated and extended until an appropriate diameter isattained. In the case where an oxygen/hydrogen flame is employed forheating and extending the multilayer silica glass rod 20, the surfacetreatment with the HF solution is essential for eliminating the moistureattached to the surface thereof.

Then, at step S18, the silica glass tube 16 a in the state where themultilayer silica glass rod 20 is inserted therein is heated under areduced pressure so as to unify them together (FIG. 4H). As a result ofthe foregoing steps, an optical fiber preform is obtained. To extend areduced length for optical fiber the optical fiber preform may beprovided with an outer cladding by known VAD or OVD method. Thusmanufactured optical fiber preform has a refractive index profilesimilar to that of the optical fiber shown in FIG. 2.

At step S19, this optical fiber preform is heated and drawn by a knowntechnique, so as to yield an optical fiber having a desirable refractiveindex profile. When an optical fiber preform having such a refractiveindex profile is employed, the mutual diffusion of Ge atoms from thering core region 13 to the center core region 11 and inner claddingregion 15 caused by the diffusion of F atoms in the directions oppositethereto is prevented due to the existence of the buffer layers 12, 14from occurring at the time of heating and drawing. Namely, the flowingof F atoms into the ring core region 13 and the flowing of Ge atoms fromthis region are suppressed, whereby the optical fiber can be preparedwhile the ring core region 13 is kept at a high refractive index state.Consequently, an optical fiber having favorable characteristics as adispersion-shifted fiber can be obtained.

A second manufacturing method will now be explained with reference toFIGS. 5 and 6A to 6F. FIG. 5 is a flowchart showing this secondmanufacturing method, whereas FIGS. 6A to 6F are lateral sectional viewsshowing the respective intermediate products at individual stepsthereof.

First, at step S21, a silica glass tube 10 a doped with GeO₂ is prepared(FIG. 6A). This silica glass tube 10 a is to become the ring core region13 having a high refractive index in the optical fiber, and has atubular form with an inside diameter of about 15 mm, an outside diameterof about 24 mm, and a length of about 300 mm, for example. Its GeO₂concentration is preferably 10 wt % to 30 wt %.

At step S22, undoped SiO₂ or SiO₂ doped with one or both of P and Cl isdeposited with a thickness of about 0.3 mm on the inner peripheral faceof the starting silica glass tube 10 a by a CVD technique, so as to formthe inner buffer layer 12 (FIG. 6B).

At step S23, a silica glass rod 11 a doped with F element, which is tobecome the center core region 11, is inserted into the inside of thesilica glass tube 10 a in which the buffer layer 12 (FIG. 6C) is formed.The F element concentration in the silica glass rod 11 a is 0.5 wt % to2.5 wt %. At this time, a gap may be formed between the silica glasstube 10 a and the silica glass rod 11 a. However, for yielding anoptical fiber whose core region has a low ellipticity, the gap ispreferably as small as possible. Also, before the insertion, both or oneof the silica glass tube 10 a and the silica glass rod 11 a may besurface-treated with an HF solution or heated and extended until anappropriate diameter is attained. In the case where an oxygen/hydrogenflame is utilized for heating and extending the silica glass rod 11 a,the surface treatment with the HF solution is essential for eliminatingthe moisture attached to the surface of the silica glass rod 11 a.

At step S24, the silica glass tube 10 a in the state where the silicaglass rod 11 a is inserted therein is heated so as to unify themtogether (FIG. 6D). This heating and unifying step is carried out in anatmosphere of Cl₂ gas or a mixed gas made of Cl₂ gas and O₂ gas. Thus, amultilayer silica glass rod 20 a is obtained.

At step S25, the multilayer silica glass rod 20 a is inserted into theinside of a silica glass tube 21 having a region doped with F atoms onthe inner periphery side and pure silica regions on both sides thereof(FIG. 6E). Here, the region doped with F atoms becomes the innercladding region 15, the outer pure silica region becomes the outercladding region 16, and the inner pure silica region becomes the outerbuffer layer 14. Here, before the insertion, the multilayer silica glassrod 20 a may be surface-treated with an HF solution or heated andextended until an appropriate diameter is attained. In the case where anoxygen/hydrogen flame is employed for heating and extending themultilayer silica glass rod 20 a, the surface treatment with the HFsolution is essential for eliminating the moisture attached to thesurface thereof.

At step S26, the multilayer silica glass tube 21 in the state where themultilayer silica glass rod 20 a is inserted therein is heated so as tounify them together (FIG. 6F). As a result of the foregoing steps, anoptical fiber preform is obtained. To extend a reduced length foroptical fiber the optical fiber preform may be provided with an outercladding by known VAD or OVD method. Thus manufactured optical fiberpreform has a refractive index profile similar to that of the opticalfiber shown in FIG. 2.

At step S27, the optical fiber preform thus obtained is heated and drawnby a known technique, so as to yield an optical fiber having a desirablerefractive index profile. The optical fiber preform obtained at step S26has a structure similar to that of the optical fiber obtained at stepS18 in the first manufacturing method whose flowchart is shown in FIG.3, whereby effects similar to those in the first manufacturing methodare obtained in the second manufacturing method as well. Namely, anoptical fiber having favorable characteristics as a dispersion-shiftedoptical fiber is obtained.

A third manufacturing method will now be explained with reference toFIGS. 7 and 8A to 8F. FIG. 7 is a flowchart showing this thirdmanufacturing method, whereas FIGS. 8A to 8F are lateral sectional viewsshowing the respective intermediate products at individual stepsthereof.

First, at step S31, a multiplex silica glass tube 20 b is prepared (FIG.8A). This multiplex silica glass tube 20 b is to become the ring coreregion 13 having a high refractive index in the optical fiber and theregions of buffer layers 12, 14 at its inner and outer peripheries; andis set such that its inside diameter is 14.4 mm, the inside and outsidediameters of the ring core region 13 are 15 mm and 24 mm, respectively,and the outside diameter as a whole is, for example, 30 mm. Details of amethod of making this multiplex silica glass tube will be explainedlater.

A silica glass rod 11 a doped with F element, which is to become thecenter core region 11, is inserted into the inside of the silica glasstube 20 b at step S32 (FIG. 8B), and they are heated in this insertedstate so as to be unified together at step S33 (FIG. 8C). Since thedetails of these steps are identical to those of the above-mentionedsteps S23, S24, they will not be explained here.

At step S34, the outer peripheral face of the unified silica glass rod30 is eliminated by grinding or etching with an HF solution, so as toprocess the silica glass rod 30 such that its outside diameter becomes1.1 times that of the ring core region 13 (FIG. 8D).

Thus processed silica glass rod 30 is inserted into the inside of asilica glass tube 21 having a region 15 doped with F atoms on the innerperiphery side and pure silica region on outer side thereof at step S35(FIG. 8E), and they are heated in this inserted state at step S36, so asto be unified (FIG. 8F). Since the details of these steps are identicalto those of the above-mentioned steps S25, S26, they will not beexplained here. As a result of the foregoing steps, an optical fiberpreform is obtained. Thus manufactured optical fiber preform has arefractive index profile similar to that of the optical fiber shown inFIG. 2.

At step S37, the optical fiber preform thus obtained is heated and drawnby a known technique, so as to yield an optical fiber having a desirablerefractive index profile. The optical fiber preform obtained at step S36has a structure similar to that of the optical fibers obtained by thefirst and second manufacturing methods whose flowcharts are shown inFIGS. 3 and 5, whereby effects similar to those in the first and secondmanufacturing methods are obtained in the third manufacturing method aswell. Namely, an optical fiber having favorable characteristics as adispersion-shifted optical fiber is obtained.

For making various silica glass tubes used in the individualmanufacturing methods, various techniques can be employed. For example,after a soot body is deposited on the outer periphery of a silica glassrod so as to produce a multilayer glass rod, the inner glass rod may beeliminated by boring or the like, so as to make a glass tube. Also,after a glass rod is produced by a known VAD or OVD technique, the innerglass rod may be eliminated by boring or the like, so as to make a glasstube.

In addition, piercing may be utilized to manufacture a silica glasstube. This manufacturing method will now be explained with reference toFIGS. 9A to 9C. FIGS. 9A to 9C are longitudinal sectional views showingthe respective intermediate products at individual steps of thismanufacturing method. Though an example in which a two-layer glass tubeis made will be explained here, it is also applicable to the making of asingle-layer glass tube or a multilayer glass tube having three or morelayers.

First, a known VAD technique, OVD technique, or the like is utilized soas to make a glass rod 22′ having a region 22 a in the vicinity of theaxial center and an outer peripheral region 22 b surrounding it (FIG.9A). Then, this glass rod 22′ is heated in an inactive gas atmosphere toa temperature not lower than 1500° C. which is the softening temperatureslightly lower than the melting point; and a rod 30 made of aheat-resistant material, such as tungsten, alumina, or carbon, forexample, is inserted therein along the center axis as shown in FIG. 9B,so as to plastically deform the glass rod 22′, thus forming an openingalong the center axis (piercing), whereby a silica glass tube 22 shownin FIG. 9C is obtained.

Thereafter, the surface of this silica glass pipe is dissolved with anHF solution having a concentration of 5% to 50%; or the silica glasspipe is heated to 1000° C. or higher, and processed by vapor-phaseetching with SF₆ gas or the like, so as to eliminate at least 10 μm ofthe inner peripheral face, thereby smoothing it. As a consequence, afterthe processing, a silica glass pipe 22 having an inner face roughness of10 μm or less, in which the boundary position between the inner region22 a and the outer region 22 b deviates little in the axial direction,is obtained. Here, an SiO₂—GeO₂ glass layer may be deposited on theinner peripheral face of a silica glass pipe by an MCVD technique, andthen the rod 30 may be inserted into this pipe so as to enhance theopening.

The inventors carried out experiments in which optical fibers wereproduced with different buffer layer thicknesses, and influences of thebuffer layer thickness were compared with each other. Their results willnow be reported.

First, influences of the inner buffer layer thickness were studied.Optical fibers produced according to the above-mentioned firstmanufacturing method were used for this comparative experiment, andtheir basic configurations are shown in Table 1.

TABLE 1 Type 1 Type 2 Type 3 Center core region diameter 2a 4.0 μm Ringcore region thickness t_(b) 7.5 μm Inner cladding region 12.0 μmthickness t_(c) Inner buffer layer thickness t₁ 0.001 μm to 5 μm Outerbuffer layer thickness t₂ 0.5 μm F conc. of center core region 0.5 wt %1.5 wt % 2.5 wt % GeO₂ conc. of ring core region 18 wt % F conc. ofinner cladding region 0.5 wt %

FIG. 10 summarizes the change in transmission loss in each type of theoptical fibers with respect to the thickness of the inner buffer layer,in which the abscissa indicates the varied inner buffer layer thickness,whereas the ordinate indicates the transmission loss at a wavelength of1.55 μm. As the inner buffer layer became thicker, the transmission losswas reduced. When the inner buffer layer thickness exceeded 1 μm, therewas substantially no difference in transmission loss even when the Felement concentration of center core region differed. Based on thetransmission loss alone, the inner buffer layer was preferably as thickas possible, and it was found preferable to have a thickness of at least0.01 μm.

Table 2 lists transmission characteristics of five optical fibers withdifferent inner buffer thicknesses among those of type 2 at thewavelength of 1.55 μm, whereas FIG. 11 is a graph of changes in bendingloss at the wavelength of 1.55 μm with respect to the buffer layerthickness when these optical fibers were bent at a diameter of 20 mm(called as 20φ bending loss).

TABLE 2 Inner buffer layer 0.001 0.01 0.1 1 5 Dispersion (ps/km/nm) −3.4−3.4 −3.5 −1.0 5 Dispersion slope 0.077 0.078 0.077 0.075 0.084(ps/km/nm²) Aeff (μm²) 83 84 85 89 160 20φ bending loss 0.4 0.4 0.6 5.0100 (dB/m)

It can be seen that the bending loss increases as the inner buffer layerbecomes thicker. From this viewpoint, it is preferred that the thicknessof the inner buffer layer be 5 μm or less. From the foregoing results,it has been found preferable for the inner buffer layer to have athickness of 0.01 μm to 5 μm.

Next, influences of the outer buffer layer thickness were studied.Optical fibers produced according to the above-mentioned firstmanufacturing method were also used for this comparative experiment, andtheir basic configurations are shown in Table 3.

TABLE 3 Type 4 Type 5 Type 6 Center core region diameter 2a 3.6 μm Ringcore region thickness t_(b) 1.5 μm Inner cladding region thickness 2.8μm t_(c) Inner buffer layer thickness t₁ 0.4 μm Outer buffer layerthickness t₂ 0.001 μm to 5 μm F conc. of center core region 1.5 wt %GeO₂ conc. of ring core region 17 wt % F conc. of inner cladding region0.3 wt % 0.5 wt % 1.5 wt %

FIG. 12 summarizes the change in transmission loss in each type of theoptical fibers with respect to the thickness of the outer buffer layer,in which the abscissa indicates the varied outer buffer layer thickness,whereas the ordinate indicates the transmission loss at a wavelength of1.55 μm. As with the inner buffer layer, as the outer buffer layerbecame thicker, the transmission loss was reduced. When the outer bufferlayer thickness exceeded 1 μm, there was substantially no difference intransmission loss even when the F element concentration of center coreregion differed. Based on the transmission loss alone, the outer bufferlayer was preferably as thick as possible, and it was found preferableto have a thickness of at least 0.01 μm.

Table 4 lists transmission characteristics of five optical fibers withdifferent outer buffer thicknesses among those of type 5 at thewavelength of 1.55 μm, whereas FIG. 13 is a graph of changes in bendingloss at the wavelength of 1.55 μm with respect to the outer buffer layerthickness when these optical fibers were bent at a diameter of 20 mm.

TABLE 4 Outer buffer layer 0.001 0.01 0.1 1 5 thickness (μm) Dispersion(ps/km/nm) −2.0 −1.9 −2.1 1.0 4.0 Dispersion slope 0.075 0.074 0.0750.074 0.084 (ps/km/nm²) Aeff (μm²) 75 77 75 73 78 20φ bending loss 0.30.2 0.2 0.4 0.1 (dB/m)

In the case of the outer buffer layer, though the dispersion valueincreases as the buffer layer is made thicker, no deterioration inbending loss or the like is seen, unlike the above-mentioned case of theinner buffer layer. As a consequence, it has been found that the outerbuffer layer can be made thicker if it is at least 0.01 μm.

A second embodiment of the optical fiber in accordance with the presentinvention will now be explained. FIG. 14 is a view showing across-sectional structure thereof, whereas FIG. 15 shows the refractiveindex profile thereof.

The optical fiber in accordance with the second embodiment is made ofsilica glass, and is constituted, as shown in FIGS. 14 and 15, by acenter core region 11 (having an outside diameter 2 a), a ring coreregion 13 (having an outside diameter 2 b and a thickness t_(b)), aninner cladding region 15 (having an outside diameter 2 c and a thicknesst_(c)), and an outer cladding region 16 which are concentricallyarranged successively from the center. It differs from the firstembodiment shown in FIGS. 1 and 2 in that it does not include the bufferlayers 12, 14 existing in the first embodiment, and that the radialdistribution of the refractive index in the ring core region is notuniform but attains a maximum value n₂ in a middle portion. Thisstructure is realized by radially changing the GeO₂ concentration in thering core region 13 such that the concentration is lower in the boundaryportion between the center core region 11 and inner cladding region 15and higher in the middle portion.

Here, the concentration of GeO₂ is preferably set such that, lettingC_(G)(r) [wt %] be the concentration of GeO₂ in the ring core region 13at a position separated from the center by the radius r [μm], theconcentration gradient y_(G1) [wt %·μ²] of GeO₂ in the boundary portionof the ring core region 13 with respect to the center core region 11defined by the above-mentioned equation (1): $\begin{matrix}{y_{G1} = {\int_{a}^{a + 1}{\left( {{{rC}_{G}(r)}{\exp \left( {a - r} \right)}} \right){r}}}} & (1)\end{matrix}$

is 100 wt %·μm² or less, and the concentration gradient y_(G2) [wt%·μm²] of GeO₂ in the boundary portion of the ring core region 13 withrespect to the inner cladding region 15 defined by the above-mentionedequation (3): $\begin{matrix}{y_{G2} = {\int_{b - 1}^{b}{\left( {{{rC}_{G}(r)}{\exp \left( {r - b} \right)}} \right){r}}}} & (3)\end{matrix}$

is 180 wt %·μm² or less. Here, y_(G1)=0 and y_(G2)=0 means that there isa buffer region 1 micrometers in thickness respectively.

An example of the method of making this optical fiber will be explainedin the following. FIG. 16 is a flowchart showing this manufacturingmethod, whereas FIGS. 17A to 17F are lateral sectional views showing therespective intermediate products at individual steps thereof.

First, at step S41, a starting glass tube 13 b made of GeO₂-doped silicaglass is prepared (FIG. 17A). This starting glass tube 13 b becomes thering core region 13. The GeO₂ concentration distribution is uniformwithin the glass tube 13 b, and the concentration is 10 wt % to 40 wt %.

Subsequently, at step S42, the starting glass tube 13 b is heated, so asto evaporate GeO₂ from the inner and outer peripheries thereof andeliminate GeO₂ from the regions in the vicinity of the inner and outerperipheries, thereby yielding the ring core region 13 having arefractive index profile such as that shown in FIG. 15 (FIG. 17B).

A silica glass rod 23 doped with F element, which is to become thecenter core region 11, is inserted into the inside of the starting glasstube 13 b at step S43 (FIG. 17C), and is heated at step S44 so as tounify them, thereby yielding a multilayer glass rod 24 (FIG. 17D).

Since the respective steps of S45 to S47 (FIGS. 17E and 17F) are similarto steps of S25 to S27 (FIGS. 6E and 6F) shown in FIG. 5, they will notbe explained here. Hence, an optical fiber preform having the refractiveindex profile shown in FIG. 15 is yielded, and an optical fiber isobtained when this optical fiber preform is drawn.

In this embodiment, at the time of heating and drawing, the diffusion ofGe atoms from the ring core region 13 to the center core region 11 andinner cladding region 15 is restrained since their diffusion velocitiesare suppressed because of the fact that the concentration gradient ofGeO₂ is set low in their boundary portions. Together therewith, theinflow of F atoms in the opposite directions is restrained. As a result,an optical fiber can be made while the ring core region 13 is kept at ahigh refractive index state. Consequently, an optical fiber havingfavorable characteristics as a dispersion-shifted fiber can be obtained.

As a matter of course, a known VAD technique or OVD technique may beused so as to radially change the doping GeO₂ concentration, therebyforming the ring core region 13. Further, glass may be deposited whilevarying its GeO₂ concentration onto the inner and outer peripheral facesof a glass tube having a uniform doped GeO₂ concentration, so as to forma concentration gradient region. Alternatively, a known MCVD techniquemay be used such that the doping GeO₂ concentration at the time whendepositing a synthetic glass layer on the inner face of a pure silica orfluorine-doped silica glass pipe successively becomes lower, higher, andlower, so as to form a concentration gradient region.

The inventors carried out experiments in which optical fibers wereproduced with different GeO₂ concentration gradients y_(G1), y_(G2) inthe boundary portions on both sides of the ring core region 13, andinfluences of the concentration gradients were compared with each other.Their results will now be reported.

First, influences of the GeO₂ concentration gradient y_(G1) in theboundary portion between the ring core region 13 and the center coreregion 11 were studied. Optical fibers produced according to theabove-mentioned manufacturing method were used for this comparativeexperiment. Their basic configurations are shown in Table 5.

TABLE 5 Type 7 Type 8 Type 9 Center core region diameter 2a 5.0 μm Ringcore region diameter 2b 8.3 μm Inner cladding region diameter 2c 14.0 μmconcentration gradient y_(G1) 10 to 150 wt % · μm² F conc. of centercore region 1.5 wt % GeO₂ peak conc. of ring core region 20 wt % 30 wt %40 wt % F conc. of inner cladding region 0.5 wt %

FIG. 18 summarizes the results of the comparative experiment, in whichthe abscissa indicates the varied values of GeO₂ concentration gradienty_(G1) in the inner interface of the ring core region 13, whereas theordinate indicates the transmission loss at the wavelength of 1.55 μm.Though no difference is seen in the case where the concentrationgradient y_(G1) exceeds 100 wt %·μm², the transmission loss becomessmaller as the concentration gradient y_(G1) is made lower when it isnot greater than 100 wt %·μm². From this point, it has been foundpreferable for the GeO₂ concentration gradient y_(G1) in the innerinterface of the ring core region 13 to be 100 wt %·mμ² or less.

Next, influences of the GeO₂ concentration gradient y_(G2) in theboundary portion between the ring core region 13 and the inner claddingregion 15 were studied. Optical fibers produced according to theabove-mentioned manufacturing method were used for this comparativeexperiment as well. Their basic configurations are shown in Table 6.

TABLE 6 Type 10 Type 11 Type 12 Center core region diameter 2a 4.5 μmRing core region diameter 2b 8.0 μm Inner cladding region diameter 2c13.0 μm concentration gradient y_(G2) 1 to 200 wt % · μm² F conc. ofcenter core region 1.5 wt % GeO₂ peak conc. of ring core region 20 wt %30 wt % 40 wt % F conc. of inner cladding region 1.0 wt %

FIG. 19 summarizes the results of the comparative experiment, in whichthe abscissa indicates the varied values of GeO₂ concentration gradienty_(G2) in the outer interface of the ring core region 13, whereas theordinate indicates the transmission loss at the wavelength of 1.55 μm.As with the above-mentioned concentration gradient y_(G1), though nodifference is seen in the case where the concentration gradient y_(G2)is at a predetermined value or greater, exceeding 180 wt %·μm² in thiscase, the transmission loss becomes smaller as the concentrationgradient y_(G2) is made lower when it is not greater than 180 wt %·μm².From this point, it has been found preferable for the concentrationgradient y_(G2) in the outer interface of the ring core region 13 to be180 wt %·μm² or less.

Though the foregoing two embodiments are explained as embodiments inwhich both of the ring core region and center core region or both of thering core region and inner cladding region are provided with bufferlayers, or both of the ring core region and center core region or bothof the ring core region and inner cladding region are provided withconcentration gradient regions; one of them may be provided with abuffer layer, while the other is provided with a concentration gradientregion.

A third embodiment of the optical fiber in accordance with the presentinvention will now be explained. The cross-sectional structure of thisembodiment is similar to that in the second embodiment shown in FIG. 14but differs therefrom in that it has the refractive index profile shownin FIG. 20. Namely, while the radial distribution of refractive index inthe ring core region 13 is made uniform, the radial distribution ofrefractive index in the center core region 11 is not uniform but attainsa minimum value n₁ in the center portion thereof. That is, aconcentration gradient region exists only in the inside of the ring coreregion 13. This structure is realized by gradually decreasing the Fconcentration in the center core region 11 from the center portion tothe boundary portion with respect to the ring core region 13.Preferably, this concentration distribution is set such that, lettingC_(F)(r) [wt %] be the concentration of F element at a position in thecenter core region 11 separated from the center by the radius r [μm],the concentration gradient y_(F1) [wt %·μm²] of F in the boundaryportion of the center core region 11 with respect to the ring coreregion 13 defined by the above-mentioned equation (2): $\begin{matrix}{y_{F1} = {\int_{a - 1}^{a}{\left( {{{rC}_{F}(r)}{\exp \left( {r - a} \right)}} \right){r}}}} & (2)\end{matrix}$

is 18 wt %·μm² or less. In the case of Y_(F1)=0, the thickness of innerbuffer layer is 1 μm.

This optical fiber can be made by the steps of preparing, as amultilayer silica glass tube, the ring core region 13 and the regionsoutside thereof excluding the center core region 11; inserting a silicaglass rod having a higher F-doping concentration in the center portionthereof into the inside of the glass tube; heating and unifying themtogether, so as to yield an optical fiber preform; and drawing thisoptical fiber preform.

The inventors carried out a comparative experiment in which opticalfibers were produced with different F element concentration gradientsy_(F1) in the outer boundary portion of the center core region 11, andinfluences of the concentration gradients y_(F1) were compared with eachother. The results will now be reported.

Table 7 shows basic configurations of the optical fibers used for thecomparative experiment.

TABLE 7 Type 13 Type 14 Center core region diameter 2a 5.0 μm Ring coreregion diameter 2b 8.3 μm Inner cladding region diameter 2c 14 μmConcentration gradient y_(F1) 5 to 20 wt % · μm² F peak conc. of centercore region 1.5 wt % 2.5 wt % GeO₂ conc. of ring core region 20 wt % Fconc. of inner cladding region 0.5 wt %

FIG. 21 summarizes the results of the comparative experiment, in whichthe abscissa indicates the varied values of concentration gradienty_(F1), whereas the ordinate indicates the transmission loss at thewavelength of 1.55 μm. It was found that the transmission loss wasreduced as the concentration gradient y_(F1) became lower, with itsreducing effect being greater at 18 wt %·μm² or less. Therefore, theconcentration gradient y_(F1) is preferably 18 wt %·m² or less.

A fourth embodiment of the optical fiber in accordance with the presentinvention will now be explained. The cross-sectional structure of thisembodiment is similar to that in the second embodiment shown in FIG. 14but differs therefrom in that it has the refractive index profile shownin FIG. 22. Namely, while the radial distribution of refractive index inthe ring core region 13 is made uniform, the radial distribution ofrefractive index in the inner cladding region 15 is not uniform butattains a minimum value n₃ in the outer portion thereof. That is, aconcentration gradient region exists only in the outside of the ringcore region 13. This structure is realized by gradually increasing the Fconcentration in the inner cladding region 15 from the boundary portionwith respect to the ring core region 13 to the outside. Preferably, thisconcentration distribution is set such that, letting C_(F)(r) [wt %] bethe concentration of F element at a position in the inner claddingregion 15 separated from the center by the radius r [μm], theconcentration gradient y_(F2) [wt %·μm²] of F in the boundary portion ofthe inner cladding region 15 with respect to the ring core region 13defined by the above-mentioned equation (4): $\begin{matrix}{y_{F2} = {\int_{b}^{b + 1}{\left( {{{rC}_{F}(r)}{\exp \left( {b - r} \right)}} \right){r}}}} & (4)\end{matrix}$

is 30 wt %·μm² or less. In the case of y_(F2)=0, the thickness of innerbuffer layer is 1 μm.

This optical fiber can be made by the steps of preparing, as amultilayer silica glass rod, the ring core region 13 and region insidethereof; inserting it into a silica glass tube having an F-doped regionon the inner side thereof with its concentration distribution becominglower in the inner side; heating and unifying them together, so as toyield an optical fiber preform; and drawing this optical fiber preform.

The inventors carried out a comparative experiment in which opticalfibers were produced with different F element concentration gradientsy_(F2) in the inner boundary portion of the inner cladding region 15,and influences of the concentration gradients y_(F2) were compared witheach other. The results will now be reported.

Table 8 shows basic configurations of the optical fibers used for thecomparative experiment.

TABLE 8 Type 15 Type 16 Center core region diameter 2a 4.6 μm Ring coreregion diameter 2b 8.0 μm Inner cladding region diameter 2c 14 μm Fconc. of center core region 1.5 wt % GeO₂ conc. of ring core region 20wt % concentration gradient y_(F2) 5 to 40 wt % · μm² F peak conc. ofinner cladding region 1.5 wt % 2.5 wt %

FIG. 23 summarizes the results of the comparative experiment, in whichthe abscissa indicates the varied values of concentration gradienty_(F2), whereas the ordinate indicates the transmission loss at thewavelength of 1.55 μm. It was found that the transmission loss wasreduced as the concentration gradient y_(F2) became lower, with itsreducing effect being greater at 30 wt %·μm² or less. Therefore, theconcentration gradient y_(F2) is preferably 30 wt %·μ² or less.

The buffer layers or concentration gradient regions formed on both sidesof the ring core region can produce various types of optical fibers bycombining the foregoing embodiments.

Further, the ring core region 13 is not restricted to a single one shownin FIG. 1, 14, 20, or 22, but a plurality of ring core regions 13 havinga higher refractive index may be disposed in a multilayer form as shownin FIG. 24A or 24B. Preferably, in this case, buffer layers 12, 14 orconcentration gradient regions are disposed between the individualGeO₂-doped higher refractive index regions 13 and F-doped low refractiveindex regions 11 in the ring core regions.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

What is claimed is:
 1. An optical fiber, composed of silica glass,having a center core region, a ring core region doped with germaniumdioxide and has a maximum concentration thereof, and a cladding region,all of which are arranged concentrically, the refractive index of saidcenter core region being lower than that of said ring core region;wherein a region located adjacent to said ring core region is doped withfluorine element and having a maximum concentration thereof; and saidregion doped with fluorine element has a boundary portion at an edge ofsaid ring core region, the thickness of said boundary portion is atleast 1 μm, the boundary portion has a concentration of germaniumdioxide that is less than the maximum concentration of germanium dioxidein said ring core region, and the concentration of fluorine element insaid boundary portion is less than the maximum concentration of fluorinein said region doped with fluorine.
 2. An optical fiber according toclaim 1, wherein said boundary portion is located in said ring coreregion, and letting a [μm] be the radius of said center core region, andC_(G)(r) [wt %] be the concentration of germanium dioxide in said ringcore region at a position separated from the center by a radius r [μm],the concentration gradient y_(G1)[wt %·μm²] of germanium dioxide in saidboundary portion defined by: $\begin{matrix}{y_{G1} = {\int_{a}^{a + 1}{\left( {{{rC}_{G}(r)}{\exp \left( {a - r} \right)}} \right){r}}}} & (1)\end{matrix}$

is 100 wt %·μm² or less.
 3. An optical fiber according to claim 1,wherein said boundary portion is located in said center core region, andletting a [μm] be the radius of said center core region, and C_(F)(r)[wt %] be the concentration of fluorine element in said center coreregion at a position separated from the center by a radius r [μm], theconcentration gradient y_(F1) [wt %·μm²] of fluorine element in saidboundary portion defined by: $\begin{matrix}{y_{F1} = {\int_{a - 1}^{a}{\left( {{{rC}_{F}(r)}{\exp \left( {r - a} \right)}} \right){r}}}} & (2)\end{matrix}$

is 18 wt %·μm² or less.
 4. An optical fiber according to claim 1,further comprising an inner cladding region doped with fluorine, whereinsaid boundary portion is located in said ring core region, and letting b[μm] be the radius of said ring core region, and C_(G)(r) [wt %] be theconcentration of germanium dioxide in said ring core region at aposition separated from the center by a radius r [μm], the concentrationgradient y_(G2) [wt %·μm²] of germanium dioxide in said boundary portiondefined by: $\begin{matrix}{y_{G2} = {\int_{b - 1}^{b}{\left( {{{rC}_{G}(r)}{\exp \left( {r - b} \right)}} \right){r}}}} & (3)\end{matrix}$

is 180 wt %·μm² or less.
 5. An optical fiber according to claim 1,further comprising an inner cladding region doped with fluorine, whereinsaid boundary portion is located in said inner cladding region, andletting b [μm] be the radius of said ring core region, and C_(F)(r) [wt%] be the concentration of fluorine element in said inner claddingregion at a position separated from the center by a radius r [μm], theconcentration gradient y_(F2) [wt %·μm²] of fluorine element in aboundary portion of said inner cladding region with respect to said ringcore region defined by: $\begin{matrix}{y_{F2} = {\int_{b}^{b + 1}{\left( {{{rC}_{F}(r)}{\exp \left( {b - r} \right)}} \right){r}}}} & (4)\end{matrix}$

is 30 wt %·μm² or less.
 6. An optical fiber according to claim 1,wherein a chromatic dispersion at the wavelength of 1.55 μm of saidoptical fiber is within the range from −1.9 to 4.0 ps/km/nm.
 7. Anoptical fiber according to claim 1, wherein a transmission loss at thewavelength of 1.55 μm of said optical fiber is 0.25 dB/km or less.
 8. Anoptical fiber according to claim 1, wherein a transmission loss at thewavelength of 1.55 μm of said optical fiber is 0.20 dB/km or more.